1. Field of the Invention
The present invention relates to an optical information-reproducing apparatus for recording data signals, such as image data signals, on a recording medium, and for reproducing the data signals from the recording medium. More particularly, the invention relates to an optical information-reproducing apparatus of the three-beam type which astigmatizes light beams by using a plane-parallel plate.
2. Background of the Invention
A conventional three-beam type optical information-reproducing apparatus is shown in FIGS. 4 and 5. The optical information-reproducing apparatus is composed of a light source 1, a diffraction grating 2, a plane-parallel plate 3, an objective lens 4, a concave lens 7, and a photo detector 8. The light source 1, which may be a semiconductor laser, emits a light beam for reading data from an optical disk 100. The diffraction grating 2 splits a light beam emitted from the light source 1 into three light beams. The plane-parallel plate 3 reflects the three light beams toward the optical disk 100, and receives signal light beams reflected from the optical disk 100, astigmatizes the received signal light beams, and emits them outwardly towards the concave lens 7. The objective lens 4 receives the three signal light beams reflected by the plane-parallel plate 3, and forms an image on the recording surface 101 of the optical disk 100. The concave lens 7 receives the three signal light beams emitted from the plane-parallel plate 3, expands the spaces (to be described below with respect to FIG. 5) between the three light beams, and forms the image on the surface of the photo detector 8.
The operation of the optical information-reproducing apparatus thus constructed will now be described. The light beam emitted from the light source 1 is diffracted in the three directions of 0th order,+1st order, and-1st order by means of the diffraction grating 2. In other words, the light beam is split into three light beams. The three light beams are all reflected on one of the major surfaces of the plane-parallel plate 3, and are projected onto the recording surface 101 of the optical disk 100 through the objective 4. The three light beams falling on the recording surface 101 of the optical disk are then modulated in intensity according to the different reflectivities of the recording surface. The light beams reflected from the recording surface are thus intensity modulated in accordance with the recorded information. That is, the light beams are transformed into signal light. The signal light beams enter the plane-parallel plate 3 through the objective 4. In the plane-parallel plate, the signal light beams are astigmatized and then emanate from the plane-parallel plate. The signal light beams are incident on the concave lens 7 at such an angle that the beams are focused in the front focal plane 41 of the objective 4 (see FIG. 5). The signal light beams, after passing through the concave lens 7, are focused on the light sensing surface of the photo detector 8.
If the concave lens 7 is not used, the three signal light beams are spaced by narrow distances L1 and are projected on the front focal plane 41 spaced a vertical distance L.sub.3 from the photo detector 8. With use of the concave lens 7, the diffusion characteristic of the concave lens 7, which depends on the magnification of the concave lens 7, acts on the signal light beams so that they are spaced by expanded distances L2 (L2&gt;L1) and are projected onto the light sensing surface of the photo detector 8. Usually, two-division photo diodes (2D-PD) 82 and 83 and a four-division photo diode (4D-PD) 81 are formed in the light sensing surface of the photo detector. Accordingly, the three signal light beams are imaged exactly in these photo diodes. Such an arrangement is described in Japanese Patent Laid-Open Publication No. Sho. 57-205833.
A conventional optical information-reproducing apparatus not using the concave lens 7 is disclosed in Japanese Patent Laid-Open Publication No. Sho. 63-4435. In this apparatus, the light sensing surface of the photo detector 8 is coincident with the front focal plane 41 of the objective 4. The 4D-PD 81 and the 2D-PDs 82 and 83 are microfabricated in the light sensing surface of the photo detector 8 so that the three signal light beams, spaced by narrow distances, will exactly fall on these photo diodes.
The first conventional device described above requires the provision of the concave lens in order to expand the beam-to-beam distance of the three signal light beams. Further, the optical path is necessarily extended by a length L3 which depends on the length of the concave lens (FIG. 5). These facts make it difficult to obtain an overall size reduction of the apparatus.
In the second conventional device described above, a plurality of photo diodes must be microfabricated in the surface of the photo detector in a high precision manner. This requires a high order of complexity in the microfabricating technique as compared to the first conventional device. Further, with this device the beam-to-beam distance is greatly limited, thus restricting the manufacturing freedom.